1. Field of the Invention
The present invention is directed to a method and apparatus for use in the photolithographic arts, particularly the semiconductor fabrication arts.
2. Description of the Related Art
In the fabrication of semiconductor devices, photolithographic masks are used to transfer circuitry patterns to silicon wafers in the creation of integrated circuits in general, in the production of semiconductor circuit devices, a series of such masks are employed in a preset sequence. Each photolithographic mask includes an intricate pattern of CAD-generated geometric patterns corresponding to the circuit components to be integrated onto the wafer. Each mask in the series is used to transfer its corresponding pattern onto a photosensitive layer (photoresist) which has been previously deposited on the silicon wafer. The transfer of the mask pattern onto the photosensitive layer or photoresist is currently performed by an optical exposure tool, which directs light or radiation through the mask to the photoresist.
Fabrication of the photolithographic mask follows a set of predetermined design rules which are set by processing and design limitations. For example, these design rules define the space tolerance between devices or interconnecting lines, and the width of the lines themselves, to ensure that the devices or lines do not overlap or interact with one another in undesirable ways. The design rule limitation is referred to within the industry as the xe2x80x9cCDxe2x80x9d (critical dimension). The critical dimension of a circuit is defined as the smallest width of a line or the smallest space between two lines which is to be permitted in the fabrication of the chip. More often than not, the CD is determined by the resolution limit of the exposure equipment. Presently, the CD for most applications is on the order of a fraction of a micron. Because of the extremely small scale of the CD, the instrument of choice for measurement and inspection is a scanning electron microscope (SEM).
When new masks are produced, or after any change in the fabrication recipe, it is customary to form a so-called focus exposure matrix (FEM) on a test wafer in order to obtain the best exposure/focus combination for the mask, e.g., the combination of focus and exposure which results in the best resolution on the wafer, in keeping with the required CD. This is typically done by, for example, sequentially exposing a series of areas of the wafer with the pattern of the mask, while exposure and focus values are incrementally changed from one exposure location to the next. After exposure of the wafer in this fashion, one can examine the individual exposure sites, for example, to check the CD, and obtain a plot of exposure v. focus or focus v. CD and determine the area of best performance from the resulting curves. Specifically, a test wafer is exposed in a stepper while the focus is varied along one axis and the exposure is varied along the other. Thus, a matrix of images is obtained on the exposed wafer, wherein each exposure site or die has a different focus-exposure setting. Selected CDs (at various locations) in each die are measured to select the best exposure-focus setting for the particular mask.
The general procedure for determining the CD in a test wafer is as follows. First, an alignment target (which is not part of the circuitry) is included on the mask, typically at an area which will not include circuit patterns. During exposure, an image of the alignment target is transferred onto each of the dies. When the test wafer is developed and loaded into the CD measurement machine (typically a CD SEM) the operator first causes the system to acquire the alignment target of the central or reference die of the wafer. The image of this alignment target is stored in memory for reference. The operator then acquires an appropriate area for CD measurement, and designates that area to the CD machine. The machine automatically calculates a vector from the center of the alignment target to the center of the designated area. This procedure is repeated for each area which the operator wishes to measure.
The foregoing procedure can be performed in what might be designated as a xe2x80x9cteaching modexe2x80x9d of the CD SEM. Once all of the data has been input and the vectors calculated, the CD system may then be enabled for automated CD measurement as described below.
When the developed wafer is properly loaded into the CD machine, the machine moves to the first die to be inspected and searches for the alignment target using a pattern recognition (PR) algorithm, using the aforementioned stored alignment target as a reference. When a high PR score is achieved, it is considered that the alignment target has been acquired. Using the stored vector, the CD machine then moves to the designated CD measurement site and acquires an image for CD measurement, which is then performed. Following this procedure, which may be duplicated for other locations on the die, the CD machine then goes to the next die and again using the PR algorithm searches for the alignment target using the stored target as a reference. Once a high PR score is achieved, the CD machine goes to the CD measurement site using the stored vector. This process is repeated until all of the designated dies have been measured.
When the measured CD is found to be outside a pre-designated value range, it signifies that something is wrong with the stepper. However, it is not known from the CD measurement whether it is the focus or the exposure that is out of its limits. Therefore, other tests needs to be performed to determine the cause of the problem.
The present invention provides a method to rapidly determine the cause of the out of bounds CD. Specifically, the inventive method allows obtaining that information at the same time the CD is being measured, thus avoiding any reduction in the throughput of the CD-SEM.
According to an advantageous feature of the invention, a golden image of the target is stored in the CD-SEM. Then, during the FEM measurements, the target of each die is compared to the golden image, and a PR score is stored. Thus, a matrix, or a look-up table, is constructed which provides a PR score for each die with respect to the golden image. Then, during a line monitoring, when a target image is obtained it is compared to the golden image and a PR score is obtained. If the measured CD turns out to be out of bound, the PR score is checked against the PR matrix of the FEM to see the focus-exposure parameters that resulted in such a PR score. Such a matching would indicate the actual drift of the focus or exposure of the stepper.